PCB Repair: Gradius III



No sound.


Many components in the sound section had been replaced during a prior repair attempt:

Typical Konami sound system c. 1989: 007232, YM2151 and Z80

Acting on the assumption that these components were good and that the sound CPU (Z80) was working as expected, I instead focused on the communication mechanism between the main CPU and sound CPU.

Schematics for Gradius 3 are unavailable but the sound section is of a similar design to other Konami games of that era for which schematics are available (e.g. Final Round).

Data is passed to the sound CPU through an 8-bit register (74LS374 at F13). The main CPU writes a sound code into the register and then asserts the sound CPU’s /INT line. The sound CPU processes the interrupt request and reads the value from the sound register. Here are the relevant signals viewed using a logic analyzer:

Logic analyzer trace showing communication between main CPU and sound CPU

Here the main CPU writes 01h, which is latched into the register on the rising edge of ‘SOUND_LATCH_CLK’. The sound CPU interrupt line is asserted sometime afterwards:

Main CPU writes 01h into the sound register

Here is the sound CPU reading the register, during which ‘SOUND_LATCH_#OE’ is asserted. We expect to see 01h but instead the value is FFh:

FFh isn’t what we were expecting

Here is the suspect component, one of the few in the sound section which hadn’t been replaced. Ironically, it appeared to be the only non-Fujitsu 74LS IC on the entire PCB…

A 1985 date code. Interesting.


Replaced 74LS374 at F13.